The present invention generally relates to reaction apparatuses and more particularly, to a gas reaction apparatus for manufacturing base material of optical fiber and a multi-wall pipe type burner for use in the gas reaction apparatus.
In order to manufacture base material of optical fiber by employing VAD method, such a multi-wall pipe type burner as shown in FIGS. 1a and 1b is widely used. The known burner includes a body 10. The body 10 is composed of a plurality of coaxially extending metallic cylindrical pipes 18a, 18b, 18c, 18d and 18e, spacers 21a, 21b, 21c and 21d for supporting the pipes 18a to 18d, respectively and heat-resistant metallic cylindrical pipes 22a, 22b, 22c, 22d and 22e constituting forward end portions of the pipes 18a to 18e, respectively. The pipes 18a to 18e are, respectively, coupled with the pipes 22a to 22e through the spacers 21a to 21d. Gas passages 19a, 19b, 19c, 19d and 19e for combustion gas of the burner, vaporous gaseous raw material formed into glass, inert gas, etc. are defined by a bore in the central pipe 18a and gaps formed between adjacent ones of the peripheral pipes 18b to 18e, respectively. The gas passages 19a to 19e are, respectively, communicated, at rearward end portions of the pipes 18a to 18e, with pipes 20a, 20b, 20c, 20d and 20e leading to respective gas supply sources.
Conventionally, in a gas reaction apparatus for manufacturing base material of optical fiber, it has been generally so arranged so that such one or more multi-wall pipe type burners as shown in FIGS. 1a and 1b are provided and gaseous raw material of optical fiber, combustion gas, inert gas, etc. are supplied into the gas passages 19a to 19e of each of the burners. Thus, the gaseous raw material is heated to a high temperature by combustion of the combustion gas at the forward end portions of the burners so as to be subjected to flame hydrolysis into glass of fine particles. The glass of fine particles is deposited on a member so as to be formed into porous base material of optical fiber.
In the manufacture of such base material of optical fiber, uniformity of material characteristics of the base material of optcial fiber in the longitudinal direction of the base material plays an extremely important role for obtaining characteristics of optical fiber. In order to obtain optical fiber having uniform material characteristics in its longitudinal direction, it is necessary to minimize variations of concentration and flow rate of the gaseous raw material introduced into the gas reaction apparatus.
In order to meet such demand, for example, two gas reaction apparatuses shown in FIGS. 2 and 3, respectively have been used. In the prior art gas reaction apparatus of FIG. 2, flow control of carrier gas 5 is performed and a condenser 8 is employed. Namely, the carrier gas 5 is supplied at a predetermined flow rate via a pipe 3, through a flow control device 4, into a raw material vessel 1 containing liquid raw material 6 of optical fiber, which is enclosed by a constant temperature bath 2. Gaseous raw material of optical fiber heated to a temperature of the constant temperature bath 2 by the constant temperature bath 2 is carried via a pipe 9a together with the carrier gas 5 from an upper space 7 of the raw material vessel 1 to a condenser 8 held at a predetermined temperature. The gaseous raw material is cooled to the predetermined temperature of the condenser 8 by the condenser 8 so as to be formed into saturated vapor. Then, the saturated vapor of the gaseous raw material is conveyed via a pipe 9b to the multi-wall pipe type burner 10 shown in FIGS. 1a and 1b, which is provided in a reaction vessel 14. At this time, since condensation of the gaseous raw material takes place if the temperature of the pipes 9a and 9b is lower than that of the saturated vapor, the pipes 9a and 9b are heat insulated or heated. Meanwhile, gases 11 other than the gaseous raw material, such as combustion gas, etc. are supplied via a pipe 12, etc. to the burner 10. Thus, reaction of the gaseous raw material is caused by heat of combustion of the combustion gas at the forward end portion of the burner 10 so as to form the gaseous raw material into glass powder such that the glass powder is grown into longitudinally extending porous base material 15 of optical fiber.
Meanwhile, in the prior art gas reaction apparatus of FIG. 3, flow control of vapor of the liquid raw material 6 is performed. Namely, the raw material vessel containing the liquid raw material 6 is enclosed by the constant temperature bath 2. Vapor of the liquid raw material 6 which has a predetermined temperature, is supplied, as gaseous raw material of optical fiber, from the upper space 7 of the raw material vessel 1 to the multi-wall pipe tye burner 10 of the reaction vessel 14 via the pipe 9a, the flow control device 4 and the pipe 9b. Flow rate of the gaseous raw material is controlled by the flow control device 4. Meanwhile, the gases 11 other than the gaseous raw material, such as the combustion gas, etc. are supplied via the pipe 12, etc. to the burner 10 shown in FIGS. 1a and 1b. Thus, reaction of the gaseous raw material is caused by heat of combustion of the combustion gas at the forward end portion of the burner 10 so as to form the gaseous raw material into glass powder such that the glass powder is grown into the longitudinally extending porous base material 15 of optical fiber. In FIGS. 2 and 3, reference numeral 13 represents flame of the burner 10.
In the prior art gas reaction apparatuses for optical fiber shown in FIGS. 2 and 3, the gaseous raw material in vapor phase is introduced into at least one gas passage such as one of those of the burner 10. At the same time, the gases 11 such as the combustion gas, etc. are introduced into the same gas passage as that for the gaseous raw material or other gas passages than that for the gaseous raw material. However, since the gases 11 such as the combustion gas, etc. are supplied from bombs exposed to atmosphere, a case may occur in which temperatures of the gases 11 are far lower than that of the gaseous raw material. In this case, the gaseous raw material is directly cooled by the cold gases 11 when the gases 11 are introduced into the same gas passage as that for the gaseous raw material. Meanwhile, when the gases 11 are introduced into other gas passges than that for the gaseous raw material, the gaseous raw material is indirectlly cooled by the coled gases 11 through the walls separating the gas passage for the gaseous raw material from the gas passages for the gases 11. Therefore, as a result of drop of temperature of the gaseous raw material, the gaseous raw material is partially liquified and thus, amount of the gaseous raw material fed into the reaction vessel 14 decreases. Thus, the prior art gas reaction apparatuses are disadvantageous in that it becomes impossible to obtain a desired amount of the base material 15 of optical fiber. In addition, the prior art gas reaction apparatuses have such a serious drawback that temperatures of the gases 11 such as the combustion as, etc. are affected by variations in temperature of atmosphere, thereby resulting in non-uniformity of material characteristics of the base material 15 of optical fiber in the longitudinal direction of the base material 15.
In order to prevent liquefaction of the gaseous raw material, there has been conventionally employed a method in which a heater is provided around the burner 10. However, in this known method, in the case where the gaseous raw material is passed through a radially inner one or ones of the gas passages of the multi-wall pipe type burner 10, it becomes impossible to sufficiently prevent liquefaction of the gaseous raw material due to rise of concentration of the gaseous raw material in the prior art gas reaction apparatus of FIG. 3 and recent increase of the number of the gas passages of the multi-wall pipe type burner 10, e.g., from four to eight-ten.